1. Field of the Invention
This invention relates generally to bandpass filters and more particularly to solid state microwave bandpass filters which utilize both surface and bulk acoustic waves in a piezoelectric medium.
2. Description of the Prior Art
Surface acoustic wave (SAW) devices are often employed as filters or resonators in high frequency circuit applications.
The advantages of using SAW devices over other frequency control methods such as LC circuits, coaxial delay lines, or cavity resonators are high Q, low series resistance, small size and good frequency stability. SAW resonators also possess advantages over bulk acoustic wave (BAW) resonators because the latter must be cut very thin to achieve high frequencies and are consequently quite fragile.
Typically, a SAW device contains a substrate of piezoelectric material such as quartz, lithium niobate, zinc oxide, or cadmium sulfide. Input and output transducers are formed upon the substrate. Transducers convert input electrical signals to surface acoustic waves (SAWs) propagating upon the surface of the substrate and then reconvert the acoustic energy to an electric output signal. The input and output transducers are frequently configured as interdigital electrode fingers which extend from pairs of transducer pads. Interdigital transducers may be formed by depositing a thin film of electrical conductive material upon a piezoelectric substrate.
Alternating electrical potential applied to the input interdigital transducers induces mechanical stresses in the piezoelectric substrate. The resulting strains propagate away from the input transducer along the surface of the substrate in the form of surface acoustic waves. These propagating surface waves arrive at the output interdigital transducer where they are reconverted to electrical signals.
SAW devices are often used as filters in a variety of applications. Compared to LC filters, for example, SAW devices can provide a narrower passband and the SAW package occupies much less physical space then an LC filter. The SAW device passband is generally determined by the number of interdigital electrode fingers and by the spacing between the fingers. Center-to-center spacing between interdigital electrode fingers is approximately one-half the wavelength of the surface acoustic wave which is most effectively transduced. A large number of interdigital fingers provides a narrower spectrum than a small number of interdigital fingers. The present invention meets the need for SAW devices with precisely tailored bandpass characteristics. The inventive device described herein is capable of providing a narrow bandpass spectrum without requiring as many interdigital fingers as the prior art.
An article pertinent to the understanding of the present invention which discusses the propagation of both surface acoustic waves and bulk acoustic waves is: P. V. H. Sabine, "Rayleigh-Wave propagation on a periodically roughened surface," Electronics Letters Vol. 6, No. 6, 19 March 1970, pp. 149.151. The Sabine article discussed a phenomenon peculiar to surface wave propagation upon a medium with a non-flat surface. The article presents results which show that surface acoustic waves suffer sharp attenuation when traveling over a medium surface corrugated in a sinusoidal shape. The sinusoidal corrugations cause a scattering of selected surface acoustic wavelengths into bulk vibration in both longitudinal and shear modes. The bulk vibratory modes withdraw energy from selected SAW wavelength components, resulting in attenuation of those components, while permitting other spectral components to traverse the surface relatively undiminished.
Applicant's copending application entitled "Microwave Band Stop Filter" Ser. No. 732,120, filed May 9, 1985, discloses a SAW device which utilizes groups of sinusoidal corrugations positioned between input and output transducers to eliminate unwanted surface wave components by driving them into bulk vibration. The present invention is an extension of the teachings of Ser. No. 732,120.
Another article pertinent to the understanding of the present invention is: R. M. Humphryes et al., "Acoustic Bulk-Surface-Wave Transducer," Electronics Letters, Vol. 5, No. 9, 1 May, 1969, pp. 175-6. The Humphryes article descibes a physical principle complementary to the principle discussed by Sabine. Humphryes teaches that a surface acoustic wave may be generated when a periodically roughened surface is irradiated by a bulk acoustic wave.
The effectiveness of both the scattering of SAW wavelength components into bulk vibration and the generation of SAW components from bulk vibration depends upon the wavelength of the sinusoidal corrugations. For a SAW (Rayleigh) wave of wavelength .lambda..sub.R and sinusoidal corrugations of wavelength .lambda., both effective scattering and effective generation of SAW components with wavelength .lambda..sub.R occur when: EQU .alpha.=.lambda..sub.R /.lambda.
The values of .alpha. depends only upon the Poisson's ratio of the medium. Poisson's ratio is a physical constant which characterizes the behavior of a solid under stress. When a typical isotropic body is stretched in one direction, the body contracts at right angles (i.e. laterally) to the stretch.) Poisson's ratio, .sigma., is the ratio of lateral percentage contraction to longitudinal percentage extension. Poisson's ratio can also be expressed as a ratio of material elastic constants or compliances. Values of Poisson's ratio may range from 0 to 0.50. An isotropic body has a single value for Poisson's ratio. However, an anisotropic body, such as quartz or other piezoelectrics commonly employed as substrates for SAW devices, may be characterized by several Poisson's ratios. The Sabine reference presents data appropriate to isotropic media, but the applicability of the results to anisotropic media is apparent to those skilled in the art, and will be illustrated later. For simplicity, the scattering phenomenon will be discussed first in terms of an isotropic medium. In an isotropic body with Poisson's ratio, .sigma.=0.1, if a surface acoustic wave of wavelength .lambda..sub.R (Rayleigh wavelength) is incident upon sinusoidal corrugations of wavelength .lambda., strong scattering of the incident surface acoustic wave into bulk vibration will occur when .alpha.=0.5 or when .alpha.=1.7. Thus, for example, strong scattering of SAW components of wavelength .lambda..sub.R occurs when 0.5=.lambda..sub.R /.lambda., or .lambda.=2.lambda..sub.R. Sinusoidal corrugations with a wavelength, .lambda., equal to twice the wavelength .lambda..sub.R of a Rayleigh SAW wave will scatter that wave into bulk vibration in the longitudinal and shear modes. (Actually, Sabine presents scattering data in terms of the parameter .lambda..sub.S, where .lambda..sub.S is the medium shear wavelength and .lambda. is the surface corrugation wavelength. However, it is well known to those skilled in the art that the Rayleigh velocity is constrained to be only slightly less than the bulk shear velocity for isotropic media and the Rayleigh velocity is only 2% or 3% less than the slower bulk shear velocity for anisotropic media. Consequently .lambda..sub.R approximately equals .lambda..sub.S. For convenience, the invention will be discussed in terms of Rayleigh wavelengths, .lambda..sub.R. Sabine's results are merely normalized in terms of .lambda..sub.S for mathematical convenience.) The amplitude of the surface acoustic wave spectral component of wavelength .lambda..sub.R after scattering is substantially diminished. The pricise amount of scattering depends upon the number of sinusoidal corrugations encountered by the incident surface acoustic wave. Greater scattering is produced by a large number of corrugations than by a small number. For Poisson's ratio, .sigma.=0.1, in an isotropic medium, for example, the condition 0.5=.lambda..sub.R /.lambda. yields an attenuation (and consequent scattering) as great as 10 dB per corrugation wavelength. Thus, surface waves of wavelength .lambda..sub.R =.lambda./2 propagating in such a medium would be attenuated by scattering approximately 100 dB after traversing 10 corrugations.
As Poisson's ratio of the medium becomes larger, the SAW attenuation curve, illustrated by Sabine, becomes lower and broader. However, the principles of SAW scattering and generation by sinusoidal corrugations remain unchanged even for an anisotropic medium, as will be discussed below.